Peoples conquered by Rome did not become Roman citizens with the rights and privileges associated with that, generally.
In the graded levels of rights in Roman law, depending on whether they were just conquered or had treaty status, they were two or three steps below citizens of Rome.
If anything, a lot of people want to be Roman citizens after they have tried it. There is a whole war in Italy called the "Social War" over extending formal, normal citizenship to Roman allies.
See e.g. First Farmers of Europe, https://www.amazon.com/First-Farmers-Europe-Evolutionary-Per...
I don't think small tribes of wandering hunters with small farming plots can sustain that many people. Caesar ran around and laid sieges to these things regularly during the Gaullic Wars. https://en.wikipedia.org/wiki/Oppidum
The Art of Not Being Governed
https://en.wikipedia.org/wiki/The_Art_of_Not_Being_Governed
which discusses in some detail the advantages of living in "marginally" farmable hillcountry.